Air engines reportedly date back to the late 17th century, but they did not appear in any significant numbers until after Dr. Robert Stirling patented his revolutionary concept in Great Britain in 1817. Thereafter, Stirling engines and variations thereof enjoyed some small degree of success for about a century until early in the 20th century when competition from electric motors and relatively lightweight internal combustion engines relegated the rather unwieldy Stirling engine into relative obscurity and what appeared would become irretrievable obsolescence.
Then, in about 1937, the N. V. Philips organization based in The Netherlands began development work on the Stirling concept, and by about 1958 they had succeeded in building a highly fuel efficient, single cylinder, experimental engine. Thereafter they developed a four cylinder Stirling engine variation utilizing double-acting pistons, and they installed it in a bus to investigate the feasibility of Stirling powered engines for vehicular purposes.
Since then, they and many other investigative teams have engaged in research and development activities to the ultimate end of employing Stirling devices in a variety of applications. Several small commercial devices have resulted, but none for vehicular purposes.
Research does continue, however, because Stirling engines would help solve many of today's environmental and energy concerns. Stirling engines can be made to operate relatively pollution free, they will operate on essentially any heat source or fuel capable of producing a sufficient temperature, they are potentially capable of achieving a higher thermal efficiency than internal combustion engines, and overall they can perform a wider variety of functions than most other prime movers. Today, more than ever before, there is a need for solving the problems that have kept the Stirling engine out of the marketplace.
Achieving a high thermal efficiency, and at the same time a reasonable power output per unit of engine weight, is accomplished by using a "working-gas" inside the cylinders that is highly pressurized. In very high performance devices this working-gas is preferably of high conductivity characteristics, as is helium, but hydrogen is generally preferred in automotive applications because it additionally is characterized by exceptionally low viscosity. And although any desirable fluid is difficult to contain at high pressures, hydrogen is particularly troublesome.
There has been some reasonable degree of success in containing the pressurized gas inside the cylinders or "working-gas enclosures", however the means of achieving this success add considerable bulk and mechanism to what originally was an unusually simple engine configuration. Bulk increases size and cost, while added mechanism increases cost and reduces reliability. And these factors combine to make current designs non-competitive with other and better known engines.
It was precisely this discomforting aspect of Stirling design that precipitated by copending patent application in the United States of America (Ser. No. 080,566 now issued as U.S. Pat. No. 4,253,303) entitled ENGINES, AND PARTICULARLY THOSE INCORPORATING THE STIRLING CYCLE filed Oct. 1, 1979. A corresponding application was filed under the Patent Cooperation Treaty (Serial No. PCT/US80/01281) on Sept. 30, 1980. These earlier disclosures teach basic concepts in sealing Stirling devices as well as reducing their size and weight. One important aspect therein involves moving all of the interconnecting mechanism between crankshaft and pistons to various locations physically within the confines of the working-gas enclosure, this comprising the expansion space, the compression space and any connecting passages or ducts therebetween. However, many modern Stirling devices of appreciable power rating (particularly engines) operate at such high internal temperatures that certain portions of the interconnecting mechanism (such as bearings) might be adversely affected by the extreme temperatures of the working-gas if not physically removed or separated from the working-gas enclosure or enclosures, or even forcibly cooled. Furthermore, it becomes more and more desireable to provide lubrication to bearings and bearing surfaces associated with the interconnecting mechanism as power rating increases. And when lubricants are employed on those bearing surfaces, the temperature of those lubricants must be held down.
High temperatures that interfere with lubrication and the seals for containing pressure have caused particular problems for that Stirling engine variation known as the "Franchot". The Franchot features a double-acting piston in each of two parallel cylinders, these pistons being interconnected by two crankshaft cranks phased 90.degree. apart. The entrapped working-gas cannot travel to opposite sides of the piston in a given cylinder as there is no path to do so, but rather travels between cylinders. One end of one cylinder is interconnected with one end of the other cylinder, and the other ends of these two cylinders are similarly connected. One of the cylinders is so arranged that opposite ends thereof comprise both expansion spaces while opposite ends of the other cylinder comprise both compression spaces. The cylinder having both expansion spaces thus remains continuously hot, and the engineering problems incident to sealing and lubricating that cylinder and its mechanism have caused this innovative configuration to be virtually ignored for development purposes.
To make matters even more difficult, as engine power increases, more sophisticated heat transfer paraphernalia usually is employed. This heat transfer paraphernalia may include small diameter heat transfer tubes as well as regenerators housing a multiplicity of fine wires. It has been found that the working-gas will entrain lubricant to which it is exposed, and this lubricant will eventually work its way into and foul these tubes and wires so as to adversely affect heat transfer and the resulting efficiency of the engine. Thus, with high performance Stirling devices, particularly engines of significant power rating, lubricants are best isolated from the working-gas.
Much of the recent research and development efforts on the larger (and usually automotive oriented) Stirling power plants centers around that Stirling variation known alternatively as the "Rinia" or as the "Siemens". This design features four cylinders with a double-acting piston in each. By ducting the pressurized working-gas from the bottom of a first cylinder to the top of another cylinder, from the bottom of this second cylinder to the top of a third cylinder, from the bottom of this third to the top of a fourth, and from the bottom of the fourth back to the top of the first, and also by properly phasing the four cranks on the crankshaft (usually about 90.degree. apart), the separate displacer pistons previously required are entirely avoided. Instead, each piston acts both as a power piston for its own cylinder and as a displacer for the working-gas for the cylinder with which it is interconnected. By alternating between displacer and power piston, the pistons of the Rinia never get as hot as the constantly hot piston of the Franchot, and the temperature of all four pistons therefore remains reasonably uniform. These considerations contribute to why the Rinia is preferred for automotive use. A more detailed treatment of the Rinia is not deemed necessary here because the Rinia's design and operation is thoroughly described in other readily available literature.
At least five large and well known organizations have worked or continue to work to make Dr. Stirling's concept a reality for vehicles. In 1978 the Ford Motor Company unfortunately terminated an exhaustive program under a cost sharing program with the U.S. Department of Energy. At least one of their designs was a Rinia square-four cylinder arrangement having connecting rods extending from connection with the double-acting power pistons inside the pressurized cylinders to a driven swashplate outside of the pressurized cylinders. Alignment of each piston and connecting rod combination was ensured at least in part by employing a crosshead between piston and swashplate. This approach consumes considerable room. Highly developed seals are used to contain the pressurized gas in each cylinder at the interface where the connecting rod extends through the cylinder endwall. But because of the relative reciprocation between connecting rod and endwall, sealing this dynamic interface has been particularly troublesome. Ford experimented with three sliding "dynamic" seals at each connecting rod interface, but the N. V. Philips development known as the "roll sock" is generally recognized as the best solution yet devised. The roll sock can be thought of as a flexible tube doubled over on itself which has one end affixed to the connecting rod and the other end affixed to the cylinder endwall. Being rigidly affixed at each end, it is in fact a flexible "static" seal rather than a "dynamic" seal, and this is a decided advantage in the sealing of hydrogen which has an uncanny ability to transgress sliding interfaces. Yet, the intricate design required to make use of this flexible static seal unduely increases the size and complexity of the resulting engine.
As is already known, much of this size and complexity can be avoided simply by pressurizing the crankcase. With the crankcase pressure elevated, the "blow-by" or transfer of gas from working-gas chamber to crankcase is not only diminished, but it becomes relatively irrelevant because the crankcase is sealed under these circumstances.
However, for engines of considerable power rating, Professor Graham Walker points out that a pressurized crankcase becomes a dominant fraction of the total engine weight, and thus this "simple expedient must be abandoned." (See Stirling Engines by G. Walker, specifically at page 119, Oxford University Press, 1980, ISBN 0-19-856209-8.)
Contributing to the above conclusion is the earlier mentioned fact that these larger Stirling devices usually incorporate not only sophisticated heat transfer paraphernalia, but also lubricants for lubricating bearings, and these lubricants must be isolated from the working-gas to avoid fouling the heat transfer tubes and regenerators. Thus, in the larger (and particularly automotive) applications, the seal between the working-gas chambers and the lubricated bearings best remains of the static seal type to ensure that absolutely no contact between working-gas and lubricants can occur. Any cyclic transfer of working-gas by way of blow-by past the pistons must in some way be kept isolated from that space containing lubricated bearings, particularly in sophisticated designs.
This background forms much of the basis for the design philosophy behind current Stirling programs for devices of significant power rating. Yet, the mechanical arrangements resulting from attempting to apply this background to new Stirling designs simply have not resulted in a marketed Stirling engine of significant power rating. Development does continue, but after about half a century since N. V. Philips began this new era of Stirling technology, one must consider that perhaps current designs are too heavily rooted in designs for other engines and thus contain inherent or fundamental weaknesses and problems for Stirling that a patchwork approach to solving cannot fully overcome.